The present invention relates to an improved speed control device for an elevator control system employing an integrator.
A speed control device in an elevator of the general type to which the invention pertains is shown in FIGS. 1 and 2. In FIG. 1, reference numeral 1 designates a three-phase AC source, 2 a transformer, 3 a speed instruction generating device for generating a speed instruction signal 3a, 4 an adder which produces a difference signal 4a, 5 an integrator for improving the characteristics of the control system with the integrator 5 providing an output signal 5a, 6 a thyristor control device, 7 a thyristor converter, 8 a current detector providing an output signal 8a which is a current signal, 9 the armature of a DC motor, 10 the field of the DC motor, 11 the sheave of a winding machine, 12 a main cable wound on the sheave 11 with the main cable being connected to a cage 13 and a balance weight 14, and 15 a tachometer generator which is driven by the armature 9 in response to which it generates a speed signal 15a.
The difference signal 4a representing the difference between the speed instruction signal 3a and the speed signal 15a is applied to the integrator 5 as a result of which the integrator 5 produces the output signal 5a. The thyristor control device 6 receives the output signal 5a and the current signal 8a and from them controls the thyristor converter 7. The thyristor converter 7 converts the voltage of the three-phase AC source into a DC voltage which is applied to the armature 9. In this manner, a well-known static Reonard system is formed with which the speed of the armature, and correspondingly the lifting speed of the cage 13, is automatically controlled by the speed instruction signal 3a with a high accuracy.
If the voltage of the AC source 1 in the system described above is decreased for some reasons, the thyristor converter 7 cannot provide the necessary output voltage and accordingly the armature 9 will not be driven at the correct speed. This will become more apparent from the waveform diagrams of FIG. 2. As indicated in FIG. 2a, at the time instant t.sub.1, because of a decrease in the AC source voltage, the thyristor converter 7 is saturated and the correct speed, represented by the speed signal 3a, cannot be provided. As a result, the output 5a of the integrator corresponding to the difference signal 4a increases after the time instant t.sub.1 as shown in FIG. 2b. Even if the speed instruction signal 3a is decreased becoming a speed reduction instruction signal at the time instant t.sub.2, the output 5a of the integrator 5 will not decrease because of its inherent integrating characteristics until the speed instruction signal 3a becomes smaller than the speed signal 15a at the time instant t.sub.3. Furthermore, even when the integrator output 5a begins dropping, it still takes a certain period of time until it reaches the correct value so that adequate speed reduction cannot be effected satisfactorily within a sufficient time. It may then result that the cage 13 cannot be stopped at a desired floor. FIG. 2c shows the armature current or the current signal 8a.
The above-described difficulties may be overcome by a technique in which the output voltage of the transformer 2 is maintained at a high value by increasing the transformer turns ratio so that the saturation value of the thyristor converter is increased. However, this technique is disadvantageous in that the transformer 2 then becomes uneconomically large in both electrical capacity and physical size making it inconvenient to install. Moreover, increasing the voltage of the transformer 2 also undesirably reduces the power factor of the elevator control device.